The present invention relates generally to wireless communication systems and methods of operating such systems to mitigate interference with the aid of coordinated transmission and training.
Wireless communication systems serving stationary and mobile wireless subscribers are rapidly gaining popularity. Numerous system layouts and communications protocols have been developed to provide coverage in such wireless communication systems.
Currently, most wireless systems are broken up into separate coverage areas or cells. Typically, each cell has a base station equipped with an antenna for communicating with mobile or stationary wireless devices located in that cell. A cellular network consists of a number of such cells spanning the entire coverage area. The network has an assigned frequency spectrum for supporting communications between the wireless devices of subscribers and base stations in its cells. One of the constraints on a wireless communication systems is the availability of frequency spectrum. Hence, any wireless system has to be efficient in using its available frequency spectrum.
It is well-known that attenuation suffered by electromagnetic wave propagation allows wireless systems to re-use the same frequency channel in different cells. The allowable interference level between signals transmitted in the same frequency channel determines the minimum separation between cells which can be assigned the same frequency channel. In other words, frequency channel re-use patterns are dictated by the amount of Co-Channel Interference (CCI) seen by the receiving unit (either the base station or the wireless subscriber device).
As an example of frequency re-use, FIG. 1 shows a portion or a cluster 10 of a typical wireless cellular system with a 7*3 re-use schedule, i.e., spatial channel re-use factor 7 and 3 sectors using different frequency channels in each cell 12. In the 7*3 case the available frequency spectrum is divided into 21 channels or sub-channels labeled by f1, f2, . . . , f21. Frequencies f1, f2, f3 are used in cell 12A, frequencies f4, f5, f6 are used in cell 12B and so on. There is no frequency re-use within cluster 10.
FIG. 1B shows a system 14 built up of clusters 10. As can be seen, the closest cell which re-uses the same frequency channel is at least three cells away. This separation ensures that sufficient attenuation is experienced by the signals emitted in the cells of one cluster before reaching cells of the next cluster re-using the same frequencies in its cells to not impair communications. The capacity of system 14 is dictated by the bandwidth of the channels and the carrier-to-interference (C/I) ratio. The sustainable re-use structure, therefore, decides the spectral efficiency of the system which is measured in the amount of information transmitted per unit frequency per cell, commonly measured in bps/Hz/cell.
Clearly, high spectral efficiency is a desirable system characteristic. By reducing CCI the C/I ratio can be improved and the spectral efficiency increased. Specifically, improved C/I ratio yields higher per link bit rates, enables more aggressive frequency re-use structures (closer spacing between cells re-using the same frequency channels) and increases the coverage of the system.
It is known in the communication art that receiving stations equipped with antenna arrays, rather than single antennas, can improve receiver performance. Antenna arrays can both reduce multipath fading of the desired signal and suppress interfering signals or CCI. Such arrays can consequently increase both the range and capacity of wireless systems. This is true for instance of wireless cellular telephone and other mobile systems.
In mobile systems, a variety of factors cause signal corruption. These include interference from other cellular users within or near a given cell. Another source of signal degradation is multipath fading, in which the received amplitude and phase of a source varies over time. The fading rate can reach as much as 200 Hz for a mobile user traveling at 60 mph at PCS frequencies of about 1.9 GHz. In such environments, the problem is to cleanly extract the signal of the user being tracked from the collection of received noise, CCI, and desired signal portions summed at the antennas of the array.
In Fixed Wireless Access (FWA) systems, e.g., where the receiver remains stationary, signal fading rate is less than in mobile systems. In this case, the channel coherence time or the time during which the channel estimate remains stable is longer since the receiver does not move. Still, over time, channel coherence will be lost in FWA systems as well.
Antenna arrays enable the system designer to increase the total received signal power, which makes the extraction of the desired signal easier. Signal recovery techniques using adaptive antenna arrays are described in detail, e.g., in the handbook of Theodore S. Rappaport, Smart Antennas, Adaptive Arrays, Algorithms, and Wireless Position Location; and Paulraj, A. J et al., xe2x80x9cSpace-Time Processing for Wireless Communicationsxe2x80x9d, IEEE Signal Processing Magazine, November 1997, pp. 49-83.
Some of the techniques for increasing total received signal power use weighting factors to multiply the signal recovered at each antenna of the array prior to summing the weighted signals. Given that antenna arrays offer recognized, advantages including greater total received signal power, a key issue is the optimal calculation of the weighting factors used in the array. Different approaches to weight generation have been presented in the art.
If the channels of the desired and interfering signals are known, the weight generation technique that maximizes the signal-to-interference-plus-noise ratio (SINR), as well as minimizes the mean squared error (MMSE) between the output signal and the desired output signal, is the well-known Weiner-Hopf equation:
w=[Rxx]xe2x88x921rxd,
where rxd denotes the crosscorrelation of the received signal vector x with the desired signal, given by:
rxd=E[x*d],
where d is the desired signal, and Rxx is the received signal correlation matrix, which in turn is defined as:
Rxx=E[x*xT],
where the superscript * denotes complex conjugate and T denotes transpose.
Of course, this technique, also known as the beamforming approach, is only one of many. Other prior art techniques include joint detection of signal and interferers, successive interference canceling as well as space-time or space-frequency filtering and other techniques. More information about these techniques can be found in the above-cited references by Theodore Rappaport and Paulraj, A. J., as well as other publications.
Interference mitigation including CCI reduction for the purpose of increasing spectral efficiency of cellular wireless systems particularly adapted to a system using adaptive antenna arrays has been addressed in the prior art. For example, U.S. Pat. No. 5,819,168 to Golden et al. examines the problem of insufficient estimation of CCI and noise in communication channels which leads to an inability to suppress interference. In particular, Golden teaches to solve the problems associated with correct estimation of the Rxx correlation matrix by an improved strategy for determining the weighting coefficients to modify Rxx based on the ratio of interference to noise.
U.S. Pat. No. 5,933,768 to Skxc3x6ld et al. addresses the problem of interference suppression with little knowledge of the interfering signal. This is done by detecting a training sequence or other portion of the interfering signal, estimating the interferer channel and using this information in a joint demodulation receiver. The training sequences come from a finite set of known training sequences. Furthermore, the training sequences of the interferers arrive at the receiver at undetermined times. The channel estimation is performed user by user and results in poor channel estimates of the interferers since their training sequences can overlap the higher powered random data sequence of the desired user signal.
In yet another communication system as taught in U.S. Pat. No. 5,448,753 to Ahl et al. interference is avoided. This is done by coordinating the direction and transmission times of the beams such that they do not cross. In this manner interference between switched beams in a network and especially between beams from adjacent base stations can be avoided. A significant effort has to be devoted to coordination between the users and the base stations in this scheme.
Unfortunately, the above-discussed and other methods to improve spectral efficiency by CCI suppression in wireless systems including adaptive antenna array systems do not exhibit sufficiently high performance. Thus, it would be desirable to improve interference suppression in wireless systems including systems using adaptive antenna arrays. In particular, it would be desirable to improve CCI suppression such that a higher rate of frequency re-use could be employed in wireless systems.
Accordingly, it is a primary object of the present invention to provide a method to mitigate the effects of Co-Channel Interference (CCI) and a wireless system adapted to practice this method.
It is a further object of the invention to provide for a sufficient level of CCI suppression to enable a higher frequency re-use in cellular wireless systems.
Yet another object of the invention is to adapt the method for use in wireless systems employing adaptive antenna arrays to further increase CCI suppression performance.
The above objects and advantages, as well as numerous other improvements attained by the method and apparatus of the invention are pointed out below.
The objects and advantages of the invention are achieved by a method for interference mitigation in a wireless communication system having multiple transmitters and receivers. In a first embodiment of the method, at least a first transmitter and a second transmitter of the system transmit a first signal S1 and a second signal S2 respectively both at a frequency f1. One of the receivers located within a coverage area receives first and second signals S1, S2. In accordance with the method a time delay is determined between reception at a specific point in the coverage area of the first and second signals S1, S2. Then, a transmission delay xcfx84 between the transmission of the first signal S1 and the transmission of the second signal S2 is introduced such that signals S1, S2 are received coherently at that specific point in the coverage area. Because of that, signals S1, S2 are received substantially coherently or even coherently (when the point is at the location of the receiver) by the receiver. This coherent reception aids in interference mitigation.
The specific point in the coverage area can be located at the position of the receiver and can be determined by ranging. Alternatively, the distribution of the receivers in the coverage area is examined and their center of distribution is determined. The specific point in the coverage area is substantially coincident with the center of the distribution. Frequently, this point will be located on an axis of symmetry of the coverage area. For example, when the coverage area is a sector of a cell, the point can be located on the axis of symmetry of that sector.
Now, when first signal S1 is the useful signal and signal S2 is an interfering signal the method calls for estimating the channels of signals S1, S2 and applying a method of interference mitigation in recovering signal S1. Depending on the system, the method of interference mitigation can include beamforming, joint detection, successive interference canceling, space-time filtering, space-frequency filtering or any other suitable technique or combination.
To further aid in interference mitigation, it is preferable that signals S1, S2 be assigned a first and a second training pattern respectively. The training patterns are chosen to be distinguishable by the receiver. Furthermore, the patterns are selected to optimize interference mitigation. In some embodiments the patterns can also be adapted to system operating parameters such as communication traffic volume. Additionally, the training patterns can be selected based on a feedback parameter, e.g., a measure of the quality of interference mitigation, obtained from the receiver.
The present method is preferably used in wireless communication systems which re-use frequencies such that the first and second transmitters transmit signals at the same set of predetermined frequencies f1, . . . , fn. The method can be used in bidirectional communications, e.g., in the downlink and uplink.
A wireless system of the invention can re-use frequencies more aggressively. For example, in the downlink the transmitters can be base stations in two cells located in close proximity or even adjacent each other. The receiver can be a mobile or fixed wireless subscriber device. In the uplink the transmitters are typically wireless subscriber devices and the receiver can be a base station. In either case the wireless subscriber devices and the base stations can use antenna arrays to further aid in interference mitigation in accordance with known techniques.
The base stations can be controlled by a base station control, as is known. In one embodiment, the base station control is responsible for introducing the transmission delay xcfx84.
The method of the invention can be used in any cellular wireless system which takes advantage of frequency re-use and seeks to reduce CCI. The method is particularly well-suited for use in systems which employ antenna arrays in its transmitters and receivers for interference mitigation. A wireless communication system employing the method of the invention has a mechanism for determining a time delay between reception of signals at the specific point in the coverage area. It also has a coordinating mechanism for introducing the transmission delay xcfx84. The wireless system can be a Time Division Multiple Access system (TDMA), Code Division Multiple Access (CDMA), Frequency Division Multiple Access (FDMA) or other multiplex communication systems using a multiple access method or a combination of such methods.
The base station control or even the master station control of the wireless system have the necessary mechanisms or circuitry for performing the functions called for by the method, such as the coordinating mechanism for introducing transmission delay xcfx84. In addition the base station control can have a training unit for assigning the training patterns.
Preferably, an analyzer is provided for analyzing the interference between the signals at the receiver. In fact, the analyzer is preferably a part of the receiver. In any event, it is preferable that the analyzer and the training unit are in communication and that the analyzer generate a feedback parameter indicating a quality of interference mitigation. This feedback is sent to the training unit which uses it in assigning training patterns.
In another method of the invention the training patterns are assigned to the signals and the coordinated reception at the receiver is such that the training patterns are received coherently at the specific point in the coverage area and substantially coherently by the receiver. This method can be implemented in a wireless system equipped with a training unit for assigning the training patterns, as described above. A detailed description of the invention and the preferred and alternative embodiments is presented below in reference to the attached drawing figures.